This invention relates to an optical scanning system used to record images such as in a phototypesetter, non-impact printer, facsimile machine, intelligent copier or computer output to a microfilm machine. It particularly relates to a solid-state scanning system which utilizes a laser as its light source.
A laser beam is a high intensity source of light which can be focused to a very fine point. Because of these properties, it finds particular utility in a scanning system to form an image on photographic film, an electrophotographic drum as in xerography or on other photosensitive media. The finely-focused spot makes possible images of small detail and its high intensity permits rapid formation of the image thereby providing the potential for a scanning system of both high resolution and high speed.
Laser scanning systems developed in the past have used mechanical means for changing the angle of the light beam to scan across the image plane. Rotating polygonal mirrors and galvanometer driven mirrors have been commonly used. More recent scanning systems, relying upon solid-state technology, have utilized the interaction of light with acoustic signals to effect the scanning action. For instance, see U.S. Pat. No. 3,851,951 issued to Jason H. Eveleth. Such systems are also discussed in Applied Optics, Vol. 5, October, 1966, pp, 1269-1638, Proceedings IEEE, Vol. 61, No. 8, August, 1973, pp. 1073-1092, and in IEEE Transactions on Sonics and Ultrasonics, Vol. SU-24, No. 1, January, 1977, pp. 7-18. In such devices the angle of deflection of the light beam is dependent upon the frequency applied to the device, the higher the frequency the larger the angle of deflection of the light beam.
In previous scanning systems, whether scanning is accomplished mechanically or by acousto-optic techniques, a separate modulator is used to vary the intensity of the laser beam as it scans to form the image. Additional means is usually provided to move the photosensitive image plane in one direction as the laser beam scans in a second direction orthogonal to the first. In this manner a two dimensional image is formed.
Mechanical scanning systems have achieved wide scan angles and high resolution; however, they generally have been slow as well as expensive to fabricate. Some mechanical systems have experienced beam position control problems because of the inertia associated with the mirrors used.
Prior acousto-optic scanners can achieve fast scanning rates; however, they generally have lacked high resolution because of limitations on the size to which acousto-optic crystals can be grown and also because of the absorption of the acoustic energy in the acousto-optic medium. Acousto-optic scanners have also been characterized by relatively narrow scan angles.
To avoid some of the limitations on mechanical scanner motion, holograms have been used to change the configuration of the scan. Such a system is described in U.S. Pat. No. 3,630,594 to Gorog. The Gorog system, however, is primarily applicable to the formation of characters such as letters of the alphabet and not to generating large area images.
Also to overcome the speed limitations of mechanical spinners, acousto-optic beam splitters have been used in conjunction with the spinners to reduce the required rotation rate. In these systems a multiplicity of modulated beams scan across the image plane forming a number of scan lines simultaneously. Substitution of a spinning ring of holograms also has been suggested in place of a faceted mirror, thereby reducing the weight of the assembly and eliminating the difficulty in reproducing the accurate facets of the mirror. Scanning apparatus using a holographic beam deflector is described in U.S. Pat. No. 4,026,630 issued to Wollenmann.